1. Field of the Invention
The present invention relates to a sampling method for potential determination in electron beam mensuration in which a periodic signal progression is to be determined at a measuring point.
2. Description of the Prior Art
Measuring the signal progressions within integrated circuits becomes more and more difficult with an increasing degree of integration. Electrical potentials can be imaged at the surface of integrated circuits with the assistance of an electron probe. Most integrated circuits must be dynamically inspected, i.e. they must function with nominal frequency during the investigation.
It is known in the art of potential measurement of chronologically periodic operations, with the assistance of electron beam mensuration that, upon exploitation of the stroboscopy effect, such circuits which operate with nominal frequency during the investigation are imaged quasi-statically in potential contrast. Thereby, a pulsed electron beam is employed which interrogates the circuit potential at a specific measuring point at a fixed time t.sub.1 synchronously with the circuit frequency. In order to obtain a high time resolution, the pulse width .DELTA..sub.t is thereby small in comparison to the period duration of the circuit signal (stroboscopic illumination). According to the principle of the sampling oscilloscope, a phase point is selected from the periodic measured signal (cut-in time of the electron beam) and the voltage value of the phase point is determined with a secondary electron spectrometry arrangement. By slowly shifting the cut-in time (phase), the signal to be measured is sampled as a function of the phase angle (H. P. Feuerbaum et al, Quantitative Measurement with high resolution of internal waveforms on MOS-RAM's using a modified scanning electron microscope, IEEE J.SC, Vol. SC-13, No. 3 (1978) pp. 319-325, fully incorporated herein by this reference). The disadvantage of this known method is that, because of the short pulse duration of the electron probe, only a small fraction of the current in the electron probe can be used for potential measurement, since the primary electron beam is blanked out for the majority of the time. Long measuring times arise as a result.